1. Field of the Invention
The invention relates to a parison for a plastic bottle, in particular, for PET bottles producible using a stretch-blow-molding process as indicated in the preamble of claim 1. The invention also relates to a method for producing a plastic bottle from a parison in a stretch-blow-molding process as indicated in the preamble of the independent method claim.
2. Background of the Invention
Containers typically used in the past composed of tinplate or nonferrous-metal plate, glass, or even ceramic, are increasingly being displaced by containers made of plastic. Particularly in the case of packaging of free-flowing media, for example, cleaning implements, toiletries, cosmetics, automotive media, etc., it is primarily plastic containers that are being employed. The low weight and low cost clearly play a not-insignificant role in this substitution. The use of recyclable plastics and the overall positive energy balance in their production also contribute to promoting the acceptance by consumers of plastic containers, in particular, plastic bottles.
The most frequently used plastic bottles are composed of polyethylene terephthalate or PET, or copolyesters thereof that have been slightly modified, and are produced using what is known as the stretch-blow-molding method. This method involves a combination of injection molding and blow molding. Here a parison is first produced in an injection mold using the injection molding process. Recently, extrusions methods have also been proposed for the goal of producing parisons. The parison has an essentially longitudinal cylindrical body, on the longitudinal end of which a base is formed. A support ring separates the body from a neck section with a pouring aperture. This neck section typically already has the subsequent form of the bottle neck. Generally, threaded sections or the like for attaching a closing component are also formed on the outside of this neck section. In a plastic injection-molding process, the parison is released from the injection mold after is it is produced, conditioned as required, and inserted into a blow mold of a blow-molding machine in which it is finally expanded by high pressure into the desired shape and then additionally stretched by a stretching core. An injection-blow-molding method is also already known in which the blowing process is affected directly following the injection-molding of the parison. Here the parison remains on the extrusion core and part of the injection mold forms a section of the blow mold.
In the stretch-blow-molding method for pure PET and slightly modified copolyesters thereof, primarily of isophthalic acid, certain stretch ratios have become established, where the design of the parison (blank) is typically defined by a longitudinal stretch ratio of 2 to 4, and a diametric stretch ratio of 2-5. The global stretch ratio, which is defined as the product of the two stretch ratios, generally ranges here between 6 and 14. It is well known that the transition between the non-stretched neck and the stretched body of the bottle represents a weak point since the material here stretches poorly. The result is that material is often wasted in this region, which is also known as a parison shoulder, due to unnecessary accumulations of material—an extremely undesirable factor in view of the cost of the material. The poorly-stretched material in the parison shoulder often has low temperature stability and compression-tension stability. In particular in the case of increased temperatures, this can result in skewed or distended necks and similar problems. Plastic bottles from the beverage industry are especially impacted by the described set of problems, the bottles being under internal pressure from nitrogen or carbon dioxide. However, even plastic bottles used in the aerosol industry are affected by this since after all these plastic bottles when in use are exposed to internal pressure from nitrogen, carbon dioxide, propane, butane, CFC, HFC, or mixtures of the referenced gases. These problems are especially prevalent in the case of plastic bottles having a fill volume of around 50 ml up to around 660 ml, and an internal pressure of 0.5 bar up to 5 bar at temperatures of 15° C. up to around 35° C.
The problem to be solved by this invention is thus to improve a parison for the production of plastic bottles, in particular, PET bottles, in a stretch blow molding method in a way so as to remedy the described problems. The goal is to prevent material wastage resulting from local material accumulations in light of scarce resources and increased environmental awareness. The goal is to prevent the weak points in the region of the parison shoulder. The goal is to create a parison that is able to be processed in known stretch blow molding equipment. The goal is to be able to use the finished stretch-blow-molded plastic bottle in a conventional manner in bottling equipment. The required mechanical strengths and thermal stabilities of the plastic bottle produced therefrom must continue to be ensured. The goal is for the parison to be producible by mass-production equipment in the injection-molding process, or also in an extrusion process.